How can we decrease ground resistance?

Friday, January 9, 2015

Sometimes adding more ground rods and pipes will not do achieve anything to bring the ground resistance down in high resistivity areas. Especially sometimes, adding more ground rods will not do anything to bring the resistance down (the resistance stays the same).

A good understanding of soil resistivity and related testing is extremely important. Some guidelines should be established for the soil testing. Similarly the testing and spacing to determine the soil resistivity with depth is important especially if deep grounding/earthing elements are going to be used.

A higher than acceptable ground resistance would affect the safe operation of power system. Several various methods had been applied in the past to decrease the grounding resistance of the grounding system. Methods include enlarging the grounding/earthing grid, connecting the main grounding grid with an external grounding grid, increasing the burial depth of the grounding grid, utilizing natural grounding object such as steel foundations of structures, adding long vertical grounding electrodes or pipes, and changing the soils around the grounding grid with low resistivity materials and chemicals.

These methods are suitable for different geographical situations but that does not mean they are going to work without a correct soil resistivity testing or proper evaluation and without collection of acceptable data to justify their application. In fact, in a specific soil environment, two or more methods can be considered to decrease the grounding resistance effectively.

The method to add deep vertical grounding electrodes to the grounding grid is very effective in substations with small area. This method can utilize the low-resistivity soil layer and eliminate the high resistivity soil near the surface affected by freezing and low temperature. In order to decrease the grounding resistance, the explosive grounding technique was proposed to decrease the grounding resistance of grounding grids in high resistivity area. The evaluation of the explosive grounding technique as with any grounding method is also affected by cost.

One efficient way to decrease the earthing resistance of substation is to use deep ground wells electrodes. Several types of deep ground wells exist: For example you can drill 150 mm ground well and go 50 meters to 85 meters deep or more in the ground until you reach moist soil ground water or limestone... The ground well is filled with bentonite material and non-corrosive material to improve resistivity.

In order to do this type of grounding you need also to rely on enhanced or more advanced grounding program in order to calculate correctly the grounding resistance since programs such as ETAP or SKM do not have the required capability to do these calculations because the grounding resistance of the equivalent model can not be easily calculated. The grounding resistance of a deep ground well can be calculated by numerical analysis software package according soil model. The SES software package can be used.